1. Field of the Invention
This invention relates generally to metal forming operations that apply vertical force to a workpiece, and more particularly to use of a mandrel or punch and die to extrude a metal workpiece without producing a void in the component teeth.
2. Description of the Prior Art
In any cold forming process in which a punch or mandrel is forced vertically through a cylindrical workpiece, the natural flow of the workpiece material is downward toward the bottom of the workpiece and away from end where the punch first enters the workpiece. The direction of the material flow is due to the vertical forces of the press and the momentum it produces. When the teeth are being extruded by the forming process, this material flow usually leaves a void at the open end of the region where incomplete teeth are produced.
Die design can produce a counter flow, called back extrusion, but filling the voided region near the open end of the cylinder is more difficult due to the material being folded forward toward the bottom of the cylinder.
When extruding gear teeth, it is critical to move the workpiece material deep into the root of the mandrel teeth. The back extrusion process is very effective near the bottom of the gear, but near the top there is no mechanism to move the material laterally toward the axis along which the mandrel moves.
A need exists for a technique to prevent extruding incomplete teeth in the workpiece by causing flow of the workpiece material toward the central axis.